As the data areal density in hard disk drives (HDD) continuously increases because of technology improvements, the magnetoresistive (MR) sensor that is used as the read-back element in HDD is required to have increasingly better spacial resolution while maintaining a reasonable signal-to-noise ratio (SNR). The sensor is a critical component in which different magnetic states are detected by passing a sense current through the sensor and monitoring a resistance change. A common giant magnetoresistive (GMR) configuration includes two ferromagnetic layers that are separated by a non-magnetic spacer in the sensor stack. One of the ferromagnetic layers is a pinned layer wherein the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer wherein the magnetization vector can rotate in response to external magnetic fields and is aligned either parallel or anti-parallel to the magnetization in the pinned layer. The spacer may be a conductive metal such as Cu in a giant magnetoresistive (GMR) device, or a dielectric layer in a tunneling magnetoresistive (TMR) sensor.
Referring to FIG. 1a, a portion of a conventional read head 8 is shown wherein a sensor element 6 is formed between a top shield 2 and bottom shield 1, and between hard bias structures 4 that are positioned on opposite sides of the sensor. Hard bias structures 4 with a longitudinal magnetization 5 provide a biasing magnetic field on the sides of the sensor to orientate the free layer magnetization 110 (FIG. 1b) in the y-axis direction or in-plane direction. There is an insulation layer 7 to separate the sensor 6 from hard bias structures 4. The thickness of the sensor element is also referred to as the reader shield (shield to shield) spacing (RSS) 3. As sensor size becomes smaller in a cross-track direction to achieve higher areal density, it is critical to also reduce the RSS spacing (down-track direction) in order to improve bit error rate (BER).
In FIG. 1b, a conventional sensor element 6 is shown with a bottom spin valve configuration that typically has a seed layer 101, AFM layer 102, pinned layer 103, spacer 104, free layer 105 having a magnetization direction 110, and a capping layer 106 that are sequentially formed on the bottom shield (not shown). Current efforts to further increase areal data density involve developing a greater data linear density along a down-track (z-axis) direction and a higher track density along the cross-track (y-axis) direction. The AFM layer, which provides bias to the pinned layer magnetization and high temperature stability, is generally one of the thickest layers in the sensor stack. Therefore, it is difficult to reduce RSS spacing without modifying the AFM design. We have previously disclosed a scheme that places a recessed AFM layer behind the ABS plane in U.S. Pat. No. 7,952,839. According to this configuration, the AFM layer is embedded in the bottom shield without exposure to the ABS and therefore reduces RSS spacing. However, this recessed AFM design is also associated with some concerns such as shield stability near the sensor, and a morphology effect on the pinned layer that may have a well known synthetic antiferromagnetic (SyAF) structure with a antiferromagnetic coupling layer formed between two magnetic layers.
One skilled in the art also recognizes that reducing RSS spacing 3 in FIG. 1a usually means the thickness of hard bias structure 4 must decrease accordingly. As a result, a thinner hard bias structure 4 may lead to a weaker pinning field on edges of free layer 105 (FIG. 1b) and thereby yield a less stable sensor 6. Meanwhile, magneto-static coupling between the hard bias structure and top shield 2 may become greater as RSS spacing decreases which can easily cause a rotation of hard bias magnetization 5 away from a longitudinal direction in the proximity of free layer. Thus, modification of the AFM layer and hard bias layers surrounding the sensor must be carefully designed in order to avoid degrading the desired properties of the sensor stack layers and hard bias structure.
To overcome the shortcomings of the prior art and to achieve a high performance sensor with reduced RSS spacing that is compatible with high data area density devices of >1 Tb/inch2, an improved read head design is needed.